Conductive Polymer: The Achievement of Professor Shirakawa
In October 2000, good news came to the Japanese science. Professor Hideki
Shirakawa, an emeritus professor at Tsukuba University, was named the recipient
of the Nobel Prize in Chemistry (along with Professors Alan MacDiarmid
and Alan Heeger) gfor the discovery and development of conductive polymers.h
A polymer is a massive organic molecule consisting of thousands of atoms, and in this case we are talking about plastic materials. As you know, polyethylenes and styrofoams donft conduct electricity. What is the difference between them and the conductive polymers? To answer the question, we need to know how electrons work.
Electric conduction means the flow of electrons. Electrons are negatively charged minute particles which can move freely inside a metallic substance. Metals like iron and copper conduct electricity because of the movement of electrons.
Polyethylene, an example of non-conductive plastics, has the structure shown in Figure 1. The electrons in polyethylene are held too tightly to move around.
In contrast, Shirakawafs conductive polymer polyacetylene looks like the structure shown in Figure 2, which has alternating single bond (|) and double bond (=). The name is derived from the fact that the polymer is made up of multiple (poly) units of acetylene (C2H2).
Fig 2 polyacetylene
The pi electrons used in the double bonds are spread out over the long chain-like molecule. This kind of alternating bonds is called a conjugated system, and electrons move through it kind of like bucket brigade. Polyacetylene is therefore a nano-scale electric wire so to speak.
However, this isnft quite enough to conduct electricity yet. The electrons are still packed tightly like a crowded train and need more room. A bucket brigade wouldnft work if everyone was holding buckets in both hands from the start. What needs to be done is to remove some electrons to gmake vacant holesh here and there, which will make passing of electrons possible.
Specifically, this involves a process known as gdopingh, which is the addition of chemicals having electron-removing property like bromine or iodine. Doping can increase the conductivity of a plastic by a billion times, up to the level matching that of metallic materials.
Shirakawa wasnft actually the one who discovered polyacetylene, as its first synthesis had been reported back in 1955. However, the plastic had never really drawn much interest of scientists because it was available only in black powder form and was difficult to work with.
In the fall of 1967, Shirakawa, who was an assistant professor at Tokyo Institute of Technology then, was asked by one of his Korean international students if he could work on the synthesis of polyacetylene. The synthesis involves the introduction of acetylene gas into the solution of the catalyst to allow the polymerization reaction to take place in the solution. Shirakawa wrote down a known procedure on a piece of paper and gave it to the student and let him do the experiment. But to their surprise, the product wasnft anything like expected black powder, but was an elastic, silver-colored film-like material.
The accident happened because the amount of the catalyst that the student used was a thousand times more than needed (It was apparently a mistake of overlooking a unit milli-, but we donft know if it was the studentfs or the professorfs). The reaction normally proceeds slowly in the solution, but it instead worked instantly at the solution surface, giving the thin film-like product. Polymers in film form are much more suited for many experiments and tests than as powder, so this accidental discovery resulted in a tremendous acceleration of the progress in polyacetylene research. When Professor MacDiarmid visited Tokyo Institute of Technology in 1976 and saw the plastic with silvery luster, he immediately offered collaboration. Professor Shirakawa eventually moved to the United States, and went on to invent conductive plastic by doping process.
Value of the Nobel Prize
The serendipitous discovery of polyacetylene film became famous as it made media headlines a number of times as a classic episode in invention story. It is true that luck was a part of the big breakthrough, but to make the most of the luck you need to have good understanding of the value of the discovery and its cause, vision to develop further plans, and patience to carry out enormous amount of experiments. Plus, the even more important leap of coming up with the idea of doping was based on Shirakawafs own experience and knowledge. I think it should be emphasized more that he didnft win his Nobel Prize by chance.
Doped polyacetylene slowly loses conductivity by reacting with moisture in the air. Today, the weakness is no longer a problem in polyacene and polypyrrole, the improved versions of conductive polymer used practically (Figures 3,4). Weighing much less than most metals, these polymers are used in bulk products like the battery of cell phones.
Fig 3 polyacene
Fig 4 polypyrrole
It is noteworthy that the Nobel Prize was given to Shirakawa and not to the people who developed these newer polymers. In the selection of the Nobel laureates, development of original concept or principle is generally appreciated more than contributions to practical application (The Nobel Prize in Chemistry in 2002 was also awarded to Koichi Tanaka, who discovered the new concept in mass spectrometry rather than to the people who modified it into practical instrument). The reason that the Nobel Prize is considered the most prestigious award is not only because of the amount of prize money, but also because of the reliable selection criteria.
I will end with a small episode. The first Japanese winner of the Chemistry Nobel Prize was Professor Kenichi Fukui, whose work in the area of theoretical chemistry including the frontier orbital theory was recognized. Professor Fukui has been interested in the theoretical basis of conductive polymers and has been collaborating with Professor Shirakawa. The two Nobel laureate chemists have already co-authored a few papers! They differ in age, career path, and area of research, but I suppose that good scientists know how to choose good science.
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